They published details of that incident, and speculated that the sex act at the time may have been the behavior of a frustrated, sexually inexperienced seal. Or an aggressive, predatory act. Or a playful one that turned sexual.

But the new incidents, published in the study "Multiple occurrences of king penguin (Aptenodytes patagonicus) sexual harassment by Antarctic fur seals (Arctocephalus gazella)", still surprised the researchers.

"Honestly I did not expect that follow up sightings of a similar nature to that 2006 one would ever be made again, and certainly not on multiple occasions," said Nico de Bruyn, of the Mammal Research Institute at the University of Pretoria, South Africa.

Scientists routinely monitor wildlife on the island, and look out for rare and unusual behaviour.

On three separate occasions, a research team led by William A. Haddad and de Bruyn spotted young male seals sexually coercing what appeared to be healthy penguins of unknown gender.

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Two incidents occurred on Goodhope Bay, and one on Funk beach. The incident in 2006 occurred on a different beach again, called Trypot.

"This really made us sit up and take notice," said de Bruyn, of the new sightings.

All four known sexual incidents followed a common pattern. Each time a seal chased, captured and mounted the penguin. The seal then attempted copulation several times, lasting about five minutes each, with periods of rest in between.

I genuinely think the behaviour is increasing in frequency

Male and female penguins mate via an opening called a cloaca, and the seals are thought to have actually penetrated the penguins in some of the acts, which were caught on film by Haddad.

In three of the four recorded incidents the seal let the penguin go. But on one of the more recent occasions, the seal killed and ate the penguin after trying to mate with it.

The incidents are the only time pinnipeds, the group that includes seals, fur seals and sea-lions, have been known to have sex with an animal from a different biological class, in this case a mammal trying to have sex with a bird.

The scientists can only speculate about why the seals are behaving this way.

But the new observations suggest that having sex with penguins may be becoming a learned behaviour among seals on the island.

"Seals have capacity for learning - we know this from their foraging behaviour for example," explained de Bruyn.

So male seals may see each other coercing penguins, then attempt it themselves.

That might explain why the number of incidents appears to be increasing. "I genuinely think the behaviour is increasing in frequency."

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But "if this is learned behaviour, we really can’t think of what the reward may be for these young males," he adds. "Other than perhaps learning that these birds are an easier target to practice their copulatory skills."

The seals were not yet old or large enough to defend harems of female seals, explained de Bruyn.

"Perhaps it is a release of sexual frustration, given the hormonal surges during seal breeding season. It is very unlikely to be failed mate recognition - i.e. the misidentification of the penguin as a female seal.

"All in all it's difficult to say really," he admits.

All videos credited to William A. Haddad, and courtesy of Polar Biology / Springer.